The unit is self contained, comprising a dual aperture chopper blade, motor and the necessary driving electronics. Each aperture provides an independent reference output allowing simultaneous dual frequency operation (10:1 ratio) for dual-path experiments. Frequency control is by a precision internal oscillator set by a 4-digit push-button selector on the unit or by the application of an external AC reference signal. The unit is powered via an external line power supply module.

Mounting holes are provided in the base and right-hand side of the housing (viewed from the front) to allow for mounting the model 197 onto an optical bench or support post.

Quartz Crystal Frequency Accuracy and StabilityThe model 197 uses a quartz crystal oscillator as its primary frequency standard. The oscillator signal is divided down to yield the required chopper frequency, and then the motor speed is continuously adjusted to phase lock the actual chopper frequency to this required value. The result is a chopper with an output frequency as stable as any other modern frequency source.

External Frequency ControlLike many other choppers, the frequency can be controlled externally. However, unlike other units the control is via an applied TTL reference rather than an analog voltage. This means that the modulation frequency generated is exactly that required which allows this chopper to be used in conjunction with the dual reference modes offered by all our lock-in amplifiers to implement two channel source compensation experiments.